Digital halftoning is a technique for displaying a picture on a two-dimensional medium, in which small dots and a limited number of colors are used. The picture appears to consist of many colors when viewed from a proper distance. For example, a picture consisting of black and white dots can appear to display various gray levels. Digital printers, which were initially pure black and white machines with a very coarse resolution, have evolved to accommodate colors, finer resolutions, and more recently, more than one bit of information per pixel (referred to as “multi-bit” or “multi-tone”).
Screening is a type of halftoning method used commonly in practical implementations. A common binary screening method employs a matrix of thresholds replicated to the size of printable area. These replicated matrices are compared to the Continuous Tone Image (CTI) to determine which PELs are ON or OFF. The print controller receives a CTI, such as a digital picture, from a host. The print controller then uses the screening algorithm to process the CTI and convert the image into an array of pixels. The result of the screening algorithm is a bitmap where each pixel may be ON or OFF which is referred to as a Half-Tone Image (HTI). The print controller then sends the HTI to a print engine for printing.
With the prevalence of devices having multi-bit capability there is a potential to improve overall image quality of print jobs using multi-bit halftoning. Multi-bit screening enables a selection among multiple drop sizes or exposure levels at each addressable pixel. The multi-bit screen consists of array of thresholds for every drop size or exposure level. Another way of representing this screen is a Look-Up Table (LUT) which is a 3D array having planes representing each darker gray level, ranging from the pattern for gray level zero through the maximum gray level of the halftone mask. The maximum gray level is used to produce a solid, where all of the pixels are printed at the maximum output state.
Several single bit halftone screen algorithms are available that may be extended to multi-bit applications in order to produce high quality halftone images. For example, the article “Multilevel Screen Design Using Direct Binary Search,” (G. Lin and J. P. Allebach) Journal of the Optical Society of America, A19, 1969-1982 (2002) demonstrate the extension of single bit screening algorithm to multi-bit using DBS with the help of schedulers. However, these algorithms require many parameters to guide through the multi-bit screen creation.
Accordingly, an algorithm to efficiently create multi-bit halftone screen is desired.